1. The Field of the Invention
The present invention generally relates to x-ray generating devices. More particularly, the present invention relates to embodiments of an x-ray tube anode target that substantially reduces the production of off-focus x-rays.
2. The Related Technology
X-ray generating devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. Such equipment is commonly used in applications such as diagnostic and therapeutic radiology, semiconductor fabrication, joint analysis, and non-destructive materials testing. While used in a number of different applications, the basic operation of an x-ray tube is similar. In general, x-rays are produced when electrons are accelerated and impinged upon a material of a particular composition.
X-ray generating devices typically include an electron source, or cathode, and an anode disposed within an evacuated enclosure. The anode includes a target surface that is oriented to receive electrons emitted by the cathode. In operation, an electric current is applied to a filament portion of the cathode, which causes electrons to be emitted by thermionic emission. The electrons are then accelerated toward the target surface of the anode by applying a high voltage potential between the cathode and the anode. Upon striking the anode target surface, some of the resulting kinetic energy is released as electromagnetic radiation of very high frequency, i.e., x-rays.
The x-rays produced by the x-ray tube target surface are known as primary x-rays and cover a range, or spectrum, of x-ray wavelengths. Though a given x-ray tube normally produces some primary x-rays along the entire x-ray wavelength spectrum, it also produces a high number, or peak, of x-rays at one or more distinct wavelengths along the spectrum. The wavelength(s) where these x-rays peaks are produced are uniquely characteristic of the material comprising the target surface of the x-ray tube anode, and thus are known as characteristic x-rays. Anode target surface materials with high atomic numbers (xe2x80x9cZxe2x80x9d numbers) are typically employed because they produce ample numbers of characteristic x-rays. The characteristic and other primary x-rays, once produced, ultimately exit the x-ray tube through a window disposed in the evacuated enclosure, and interact in or on various material samples or patients. As is well known, the x-rays can be used for sample analysis procedures, therapeutic treatment, or in medical diagnostic applications.
One application for which x-ray tubes are well suited is referred to as x-ray fluorescence spectroscopy (xe2x80x9cXRFxe2x80x9d). XRF is typically used to determine the elemental composition of a selected material. An XRF instrument setup typically includes an analytical x-ray tube (AXT), a specimen to be analyzed, a collimator, a diffracting crystal, and an x-ray detector. To analyze the composition of the specimen, the x-ray tube is activated and x-rays are directed at the specimen. The interaction of the x-rays, particularly the characteristic x-rays, with the atoms in the specimen causes the atoms to emit, or fluoresce, a second group of excited x-rays, some of which possess wavelengths characteristic of the elements in the specimen. Once emitted by the sample, the fluoresced x-rays are dispersed into an x-ray spectrum by a diffracting crystal, then collimated towards a detector and associated instrumentation, which quantify and correlate the results. Similar to the characteristic x-ray peaks produced by the x-ray tube target material, the intensities of the various wavelength peaks in the XRF spectrum are roughly proportional to the concentration of the corresponding elements that comprise the specimen. In this way, the elemental composition of a variety of materials may be ascertained.
Many x-ray tubes employ a rotary anode that rotates portions of its target surface into and out of the stream of electrons produced by the cathode. However, analytical x-ray tubes, such as those used for XRF applications, typically use a stationary anode. The stationary anode typically includes a substrate portion, comprised of copper or similar material, and the target surface, which may comprise rhodium, palladium, tungsten, or any other suitable material. For an XRF procedure to yield superior results when assaying a specimen, it is highly desirable that the x-ray tube produce a stream of primary x-rays that is spectrally pure, i.e., the x-ray wavelength spectrum of the primary x-ray stream contains characteristic wavelength peaks that originate only from the target material disposed on the target surface of the x-ray tube anode, and not from contaminating x-ray sources.
Unfortunately, many of the electrons that impact the target surface do not produce primary x-rays. Rather, a significant number of electrons simply rebound from the anode target surface and strike other non-target surfaces within the x-ray tube, such as the anode substrate. These electrons are often referred to as xe2x80x9cback-scatteredxe2x80x9d electrons. These back-scattered electrons retain a significant amount of their original kinetic energy after rebounding. As such, these secondary collisions with non-target surfaces can produce secondary x-rays having wavelengths that are characteristic of the material impinged, such as copper. These secondary x-rays are emitted from the x-ray tube along with the primary x-rays created at the target surface of the stationary anode. In XRF spectroscopy, these secondary x-rays may be considered an undesirable contamination of the primary x-ray stream because they can interfere with the measurement of the fluorescing x-rays emanating from the specimen under analysis. In other words, an XRF detector may mistake a contaminating secondary x-ray having, for example, a characteristic copper wavelength produced by the copper anode substrate as having been produced by a fluorescing copper atom present in the specimen under analysis. Thus, to optimize the spectral purity of the signal, it is critical to reduce or eliminate contaminating secondary x-rays from the x-ray emissions of an x-ray tube.
Several attempts have been made to eliminate secondary x-ray contamination from primary x-ray emissions. One approach has involved the use of a graphite layer to cover a portion of the anode substrate where back-scattered electrons typically impact. Though this approach reduces the amount of contaminating x-rays that are emitted, it gives rise to other problems. In particular, the approach results in serious outgassing and particle creation problems during tube operation because of differing thermal expansion rates between the graphite layer and the anode substrate, and because of the extensive machining and handling steps required for assembly and attachment of the graphite layer. Outgassing and particle creation within the evacuated environment of an x-ray tube are highly detrimental to its performance and operating lifetime. Additionally, a graphite layer is relatively difficult to attach to the surface of a stationary anode.
Another approach has involved extending the target surface beyond the periphery of the anode substrate to create an overhanging ledge that serves as a barrier to electrons backscattered off of the target surface. While partially effective in blocking some backscattered electrons, the ledge may be unable to stop electrons that travel beyond the ledge and impact the anode substrate, creating contaminating secondary electrons. Moreover, a target surface having an overhanging ledge of this type may not conduct heat as efficiently as desired.
In light of the above, therefore, a need exists for a stationary x-ray tube that reduces or eliminates the production of contaminating secondary x-rays. This need is especially acute in x-ray tubes employed in XRF spectroscopy operations, which require spectrally pure x-ray streams. Further, any solution to enable the creation of spectrally pure x-ray streams should do so without creating ensuing problems, such as outgassing, particle creation, or heat conduction problems that are detrimental to the operation of the tube.
The present invention has been developed in response to the current state of the art, and in particular, in response to these and other problems and needs that have not been fully or adequately solved by currently available x-ray tubes. Briefly summarized, embodiments of the present invention are directed to an anode target cap that reduces or eliminates contaminating secondary x-ray emission in stationary anode x-ray tubes. In addition, the anode target is implemented in a manner so as to prevent other problems within the tube, such as outgassing, particle creation, and thermal retention.
The anode target cap as disclosed in preferred embodiments generally comprises a body having a planar top wall and a continuous, cylindrical side wall. The top and side walls cooperate to define a cylindrical cavity into which is received one end of the anode substrate such that the end and a portion of the substrate adjacent the end is covered.
The target cap is comprised of a material that is capable of producing x-rays, such as rhodium, palladium, or tungsten. This enables the outer surface of target cap top wall to serve as the target surface of the stationary anode. As such, the top wall of the cap is oriented to receive electrons from the cathode that strike the target surface and produce a stream of primary x-rays.
The side wall of the target cap comprises a length sufficient to cover the portion of the anode substrate that is susceptible to impingement by backscattered electrons. In this way, backscattered electrons that otherwise would impact the anode substrate instead impinge the side wall of the target cap. Because the side wall of the target cap comprises the same material as the target surface, the wavelengths of the secondary characteristic x-rays that are produced by the impingement of the backscattered electrons on the side wall are nearly identical to the wavelengths of the primary characteristic x-rays produced by the target surface. As a result, any side wall-produced secondary x-rays that exit the tube along with the primary x-ray stream do not negatively impact or interfere with the analysis being conducted with the x-ray tube. This, in turn, results in improved performance of x-ray tube, as well as more reliable analysis results, especially in applications such as XRF.
The thickness of the top and side walls of the target cap may be varied according to the particular application, but it need only be thick enough to prevent the penetration of backscattered electrons through the top or side walls. The longitudinal length of the side wall may be varied to cover as much or as little of the surface of the anode substrate as may be needed for a particular tube application. The desired side wall length is determined by several factors, including the amount of energy imparted to the electrons during their acceleration from the filament to the target surface on the target cap.
The present anode target cap makes possible the production of spectrally pure primary x-ray streams by reducing or eliminating the production of contaminating secondary x-rays. Inaccuracies created by such contamination in sensitive analysis procedures, such as XRF spectroscopy, are significantly reduced or eliminated. Therefore, the composition of samples subjected to XRF spectroscopy may be determined with greater precision that what was before possible. Moreover, the shape and design of the target cap allows for relatively greater heat dissipation from the anode substrate than what is possible using a graphite sleeve. Additionally, use of the present target cap avoids the problems associated with outgassing and particle creation encountered with prior art solutions.
These and other features of the present invention will become more fully apparent from the following description and appended claims.